Peerless-AV PB-1 Plenum Box For CMJ450/453/455 and 500This easy to install Plenum box is a universal solution for any false ceiling application. Featuring the only slide on lid design in the industry, this solution is the simplest and easiest to install into virtually any application, specializing in low ceiling environments. Features Optional internal knockouts for clean installations in any application 3" (76 mm) of lateral adjustment for precise placement " (13 mm), " (19 mm) and 1" (25 mm) knockouts